"You faintly remember the days of constant nuking in your past life. You can learn 1 Evocation spell from the Sorcerer spell line equal to your highest casting level."
Example:
You were a sorc but you TR'd into a bard.
You're currently lvl 1 so only know bard level 1 spells.
Thus, you may know 1 lvl 1 sorc Evocation spell.
You choose... Burning Hands.
At lvl 4 bard spells you change to Wall of Fire!
But, you're not spec for fire/ice because you're a bard!
Thus, you're not OP. But, it's still neat.
A bard could only learn up to a lvl 6 sorc evocation spell because their highest casting lvl is 6.
If you went 20cleric you and held a Green Blade you could crit on bludgeon damage with a Meteor Swarm, though.